1. Field of the Invention
This invention relates to heat exchangers and particularly to heat exchangers of the type comprising a bundle of tubes contained within an outer shell.
2. Description of Related Art
Such heat exchangers are used, for example, as "once-through" steam generators in liquid metal cooled fast breeder nuclear reactor power plant. In such plant, a liquid alkali metal, such as sodium, heated by the nuclear reaction, is passed through the shell in contact with the outer surface of the tubes, while water is passed through the tubes. The water is vapourised thereby, and the steam generated is used to drive one or more turbine-generator units.
A schematic sectional view of a conventional steam generator unit for a liquid metal cooled fast breeder reactor (LMCFBR) is shown in FIG. 1 of the accompanying drawings. The unit comprises a straight elongate vertical shell 1 extending between a feed water inlet header 2 and a steam outlet header 3. The header 2 has a water inlet nozzle 4 and the header 3 has a steam outlet nozzle 5. A bundle 6 of vertical tubes conducts water and steam from the header 2 to the header 3. For the sake of clarity, only the outline of the bundle is shown as two chain-dotted lines. The tubes extend between a tubeplate 7 in the header 2 and a tubeplate 8 in the header 3, and are welded at their respective ends to the tubeplates. The bundle 6 of tubes is enclosed within a cylindrical shroud 9, and is supported by horizontal grid plates 10, spaced apart over the length of the shroud.
Liquid sodium is fed into the shell 1 via an inlet nozzle 11, passes through an annular chamber 12 and a distribution grid 13 and enters the interior of the shroud 9. The sodium flows downwards within the shroud in thermal contact with the tubes, passing through the grid plates 10. The major part of the sodium flow leaves the shroud via apertures in an outlet section 14, enters an annular chamber 15 and then leaves the shell 1 via an outlet nozzle 16. The remainder of the liquid sodium flow is conducted downwards through grids 17, 18, 19 to act as a thermal barrier to protect the tubeplate 7. The shell may include a bellows device 20 to allow for differential expansion of the shell and the tubes.
This conventional type of steam generator unit suffers from a number of disadvantages. Firstly, the straight shell and tube configuration requires the bellows device to give tolerance to tube-shell temperature differences. Secondly, the configuration has poor tolerance to temperature differences between the tubes. Thirdly, it is very long (for example approximately 37 meters), and this gives rise to a number of problems. Thus, the building in which it is housed must be very high, manufacture, transport and erection of the unit are difficult and, more especially, the tubes must be in continuous lengths, because sub-sodium tube to tube welds are considered undesirable. Furthermore, the plant required to draw tubes of the full heat exchanger length and to heat treat them would involve very considerable capital expenditure.
Some of these problems have been alleviated in some known heat exhangers, such as shown in British Patent Specification No: 1,088,115, by forming the tube-in-shell arrangement into an inverted-U configuration, thereby reducing the overall height of the heat exchanger.
However, the above-mentioned prior specification discloses the use, in a boiler feedwater heater, of tubes formed in continuous lengths, so the overall tube lengths which have to be manufactured are large. It is not suggested therein that the tubes should be formed in shorter lengths which are then welded together during assembly of the heat exchanger.